Previous brushholder constructions were of two basic types. The first type employs a brush tube with both ends open. The brush assembly, comprising a carbon block, shunt, spring, and contact terminal is inserted into the tube from the end away from the commutator or slip ring. The terminal is clamped into place using a brush cap, clip, pin or other secondary part. This clamping anchors the shunt so that the brush travel can be limited, and it provides an electric contact between the terminal and the brushholder or other conductor.
The second type requires the brush to be loaded into the tube from the commutator or slip ring end. When shunted brushes have been used in these dead-ended tubes, something had to be pushed through a hole in the tube to hold back the spring or terminal in order to limit brush travel. This type of unit will retain the brush and the spring only after the secondary connection is made.